In a hospital setting there is always the need to monitor patient health through the evaluation of blood chemistry profile. The simplest method employed in the hospital is to use a syringe carrying a sharpened cannula at one end and insert that cannula into a vein or artery to extract a blood sample from the patient. Patients that are in the critical care units or operating room sometimes require as many as twelve samples a day. Such frequent sampling injections potentially expose the patient to airborne bacteria and viruses which can enter the bloodstream through the opening made by the sharpened cannula. Furthermore, accidental needle sticks of the nurse or technician frequently occur. The problem of infection, accidental needle sticks and ubiquitous danger of contracting viruses such as HIV or hepatitis has prompted the medical field to adopt alternative blood sampling systems.
One way to obtain a blood sample is to draw the blood from a catheter that is already inserted in the patient, either in a central venous line, such as one placed in the right atrium, or in an arterial line. Typically, existing injection sites for arterial or venous drug infusion or pressure monitoring lines are used to take periodic blood samples from the patient. Conventional mechanisms for drawing blood from the lines used for infusion or pressure monitoring utilize a plurality of stopcock mechanisms that preclude flow from the infusion fluid supply or from the pressure column drip supply, while allowing blood to flow from the patient into a collecting syringe connected to a removal port formed in one of the stopcocks. However, stopcocks increase contamination and risk of infection and increase the blood waste. Furthermore, the earliest uses of such sampling sites were with sharpened cannula forced through an elastomeric septum provided in a port in the stopcock housing. Repetitive piercing of the septum at such injection sites were a source of physical damage known as coring or laceration which could shorten the effective life of the injection site. Furthermore, such an apparatus failed to eliminate the danger of the nurse or clinician being stuck by the sharpened cannula.
The next development in sampling systems was to use blunt cannula and slit septums. Although the blunt cannula removed the danger of sticking the nurse or clinician, the possibility of infection by blood borne pathogens remained since the blood at the sampling site and in the syringe was typically under arterial or venous pressure, and in extreme cases could cause a fine spray of blood to contact the user.
Most early systems also required a two-step operation where a first sample of fluid, generally about 5 ml in volume for intensive care environments was withdrawn into the sampling syringe and discarded. This first sample potentially included some of the infusion fluid and thus would be an unreliable blood chemistry measurement sample. After the initial sample had been discharged, the second sample was pure blood from the artery or vein. Aside from the unnecessary loss of blood, the two-sample process potentially introduced undesirable effects relating to such problems as introduction of air into the arterial line and introduction of contaminants into the blood supply. The two-step process also requires substantial effort on the part of the nurses or other clinicians who must draw the blood sample.
In response to the drawbacks associated with earlier sampling systems, closed systems were developed, such as the blood sampler device in U.S. Pat. No. 4,673,386 to Gordon. The Gordon device is shown schematically in FIG. 1 and comprises a piston/chamber device 20 positioned in an infusion line upstream of a sampling port 22. The sampling port 22 includes a slit septum 24 into which a blunt cannula 26 may be inserted for sampling blood therefrom. In use, the piston in the device 20 is retracted to pull fluid from the patient towards the fluid supply and store it in the chamber. Enough fluid is withdrawn into the chamber to pull a pure blood supply past the sampling port 22 so that a syringe 28 can withdraw a usable sample of blood through the blunt cannula 26. In some systems which make use of the Gordon device, such as the Edwards Critical-Care Venous Arterial Blood Management Protection System (VAMP.TM.), a shut-off valve is positioned between the piston/chamber device 20 and sampling port 22 to insure that the syringe 28 does not pull any of the dilute blood or infusion solution from within the chamber. Such closed systems eliminate the problem of needle sticks, and reduce the number of injections into the patient to one for the initial cannula introduction. The process still requires a two-handed operation on the piston/chamber device 20, followed by the two-handed operation of the syringe 28 to obtain the sample of blood.
As mentioned above, blood samples can also be taken from catheter lines used for monitoring blood pressure. The closed system of Gordon shows a pressure transducer 30 placed in the infusion line connected to a monitor 32 which displays the blood pressure. Such pressure lines typically make use of relatively stiff tubing primed with a suitable IV fluid such as saline or 5% dextrose solution as a pressure column. For adults, a bag pressurized with air surrounds an IV fluid supply bag to maintain a constant pressure differential in the line constantly urging fluid toward the patient through a restrictor orifice. The slow drip of IV fluid flushes the line to prevent clotting. The transducer includes a diaphragm exposed to the pressure column on one side and having a device for measuring deflection of the diaphragm on the other. Some transducers such as the TruWave.TM. Disposable Pressure Transducer available from Baxter Healthcare of Irvine California include a flush device that also can be used for sending transient pressure waves through the line. A Snap-Tab.TM. device of the TruWave.TM. is a rubber tab which when pulled and then released sends a square wave through the pressure column which can be used to check the inherent frequency response of the entire system, which includes the tubing and any components attached thereto, such as the sampling ports and temporary fluid storage devices. Proper system frequency response is necessary for reliable blood pressure measurements.
Another closed sampling system manufactured by Abbott Laboratories, and disclosed in U.S. Pat. No. 5,324,266 to Ambrisco, et al., is seen in FIGS. 2A and 2B. This system includes a fluid supply 34 connected through a conduit 36 to a variable flow control device and flush valve 38. The flush valve is connected to the proximal end of a fluid storage mechanism 40 having a piston 42 therein with a hollow interior 44. Infusion fluid from the supply 34 drips through the hollow interior 44 as indicated by arrow 46 and through a vortex inducer element 48 out of the storage mechanism 40. The infusion fluid continues through the line 36 past a shut-off valve 50, a sampling port 52 and finally through a sharpened cannula 54 which has previously been implanted in the patient. When a sample of blood is required, the user grips a lock cap 56 and squeezes the two sides, releasing it from a flange 58. As the cap 56 is withdrawn, the piston 42 creates a vacuum within the fluid storage mechanism 40 pulling blood and residual infusion fluid from the patient into a chamber 60 (FIG. 2B). At this point, the shut-off valve 50 is closed, and a blunt-tipped cannula is used for pulling a sample of blood from a sampling port 52. Although the device in the Ambrisco patent is purportedly easier to use than the Gordon device because of its concentric filling chamber configuration, a two-handed operation to pull fluid within the chamber 60 and a two-handed operation to draw blood from the sampling port 52 are still required.
In view of the foregoing, there is a need for a simplified blood sampling system and method.